Accumulated Other Comprehensive (Loss) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	$ (8,361)	$ 23,880	$ 104,547
Other comprehensive income (loss) before reclassifications	(80,500)	1,921	55,073
Amounts reclassified from accumulated other comprehensive income (loss)	86,710	(34,162)	(135,740)
Net current-period other comprehensive income (loss)	6,210	(32,241)	(80,667)
Balance at end of period	(2,151)	(8,361)	23,880
Net Gains (Losses) on Cash Flow Hedges
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	(6,551)	30,717	121,358
Other comprehensive income (loss) before reclassifications	(80,175)	(2,720)	53,815
Amounts reclassified from accumulated other comprehensive income (loss)	86,726	(34,548)	(144,456)
Net current-period other comprehensive income (loss)	6,551	(37,268)	(90,641)
Balance at end of period	0	(6,551)	30,717
Defined Benefit Pension and Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component, net of tax
Balance at beginning of period	(1,810)	(6,837)	(16,811)
Other comprehensive income (loss) before reclassifications	(325)	4,641	1,258
Amounts reclassified from accumulated other comprehensive income (loss)	(16)	386	8,716
Net current-period other comprehensive income (loss)	(341)	5,027	9,974
Balance at end of period	$ (2,151)	$ (1,810)	$ (6,837)